Effect of head position on intraocular pressure in horses.

OBJECTIVE To evaluate the effect of head position on intraocular pressure (IOP) in horses. ANIMALS 30 horses. PROCEDURES Horses were sedated with detomidine HCl (0.01 mg/kg, IV). Auriculopalpebral nerve blocks were applied bilaterally with 2% lidocaine HCl. The corneas of both eyes were anesthetized with ophthalmic 0.5% proparacaine solution. Intraocular pressures were measured with an applanation tonometer with the head positioned below and above heart level. The mean of 3 readings was taken for each eye at each position for data analysis. The effect of head position on IOP was assessed and generalized estimating equations were used to adjust for the correlation from repeated measures of the same eye and intereye correlation from the same horse. RESULTS Of the 60 eyes, 52 (87%) had increased IOP when measured below the heart level. A significant difference (mean +/- SE, 8.20 +/- 1.01 mm Hg) was seen in the mean IOP when the head was above (17.5 +/- 0.8 mm Hg) or below (25.7 +/- 1.2 mm Hg) heart level. No significant effect of sex, age, or neck length on IOP change was found. CONCLUSIONS AND CLINICAL RELEVANCE Head position has a significant effect on the IOP of horses. Failure to maintain a consistent head position between IOP measurements could potentially prevent the meaningful interpretation of perceived aberrations or changes in IOP.